1. Field of the Invention
This invention relates generally to lamp dimmers, and more particularly to a universal dimmer which can be used with either fluorescent or incandescent lamps powered by alternating current or direct current.
2. Brief Description of the Prior Art
Most commercially available light dimmers utilize a switching circuit in series with the lights to be dimmed. These types of dimmers reduce the percentage of time the lights are on. For example, from 100% (full on) down to 0% (off). Their circuitry is designed to vary the electrical conduction angle. The electrical conduction angle is defined as the 360 degrees of a full cycle of 60 Hz (or other frequency) power line alternating voltage (usually sinusoidal). This is usually accomplished by either of two semiconductor switching devices known as a "triac" and a "silicon controlled rectifier" (SCR). Use of these switching devices demands the use of alternating current. They cannot cut off direct current.
The commercially available dimmers for incandescent lamps carry a warning against their use with fluorescent light fixtures. The reason being that fluorescent lamps have an inductive ballast in series with the lamp to limit current to a safe level. The current in an inductor cannot be changed instantaneously, and the switching devices of the incandescent lamp dimmers will produce large voltage spikes in the inductive ballast of an fluorescent lamp. They would also produce electromagnetic noise which would interfere with radio and television reception.
The common fluorescent lamp is a member of a family of very efficient light sources known as arc discharge lamps. The family of arc discharge lamps includes; fluorescents, mercury vapor, sodium vapor, xenon, and the so-called "neon" lamp. True neon is a generally red color, the other colors are some other type of gas. The mercury vapor lamps (blue-white) and sodium vapor lamps (orange) are commonly used as street lights and freeway lighting. All of these light sources require a high voltage to break down the conducting gas and, once the arc is established, there is nothing internal to the lamp to limit the current. Limiting of the current is accomplished with an external device known as a ballast. Most ballasts are of the inductive type to prevent dissipation of any more energy than is necessary.
The common fluorescent lamp is basically a mercury vapor lamp which some additional features. The fluorescent lamp differs from a mercury vapor lamp in that the mercury vapor lamp utilizes the visible light generated by the excited mercury gas, whereas the fluorescent lamp has a phosphor coating on the inside of the fluorescent tube which is made to glow as a result of the mercury light emission generated by the electrical arc. The mercury gas has a strong ultraviolet (UV) line which is not visible, and the phosphor coating in the fluorescent tube is excited to glow by the ultraviolet emission line. The light from the excited phosphor is added to the visible light emitted by the mercury vapor.
In all arc discharge lamps, the amount of light generated is a function of the current in the arc. The same is true of incandescent lamps, however, the incandescent lamp is a resistance device. Thus, to dim an incandescent lamp, one may limit the duty cycle of the current or limit the current, and to dim a fluorescent lamp, one must limit the current only, and in a smooth manner.
Electronic high frequency "switching ballasts" have recently been developed which, similar to the "SCR" type dimmers, limit the current conduction duty cycle. In the switching ballasts, the switching device is the ballast and there is no inductor where the current is switched in an uncontrolled manner. The switching ballasts are still too expensive for consumer use.
Special dimming circuits are available for fluorescent lamps which require special ballasts, special wiring, and a special control which mounts on the wall. The fixture is wired quite differently than the conventional home and office fixture. The special ballast is wired such that the voltage on the filaments extending between the two pins at each end of the tube is maintained constant while the current through the lamp is varied.
U.S. Pat. Nos. 3,935,505, 3,819,982, 3,614,527, and 3,358,187 disclose dimmers of the type which utilize a "triac" or a "silicon controlled rectifier" (SCR) semiconductor switching device and are designed to vary the electrical conduction angle. These types of dimmers cannot be used with conventional fluorescent ballasts.
U.S. Pat. No. 4,172,981 discloses a dimmer having a blocking oscillator to generate high frequency excitation and uses a transformer to step up the voltage to a level sufficient to arc the lamp. This device will not work with conventional ballasts and does not operate at the standard 60 Hz line frequency.
U.S. Pat. No. 3,264,518 discloses a dimmer which utilizes a saturable core reactor as the control element. The saturable core reactor is a transformer with DC windings which are used to saturate the magnetic core of the transformer and vary the inductive reactance. The variable source of DC current is supplied by SCR circuits.
The present invention is distinguished over the prior art in general, and these patents in particular by a universal dimmer which can be used for both fluorescent lamps having a ballast and for incandescent lamps connected to either an alternating or direct current power supply. The dimmer can be installed in place of a standard wall-mounted light switch and connected to the existing wiring. The dimmer circuitry has a fullwave diode rectifier bridge connected in series with the lamp for converting the alternating current power into direct current. A variable current limiting circuit is connected in series with the DC output of the rectifier bridge and is powered by direct current to control the amount of current supplied through the lamp and ballast. The current limiting circuit includes a power transistor which acts as a variable resistor and controls the lamp current and an operational amplifier coupled with a voltage divider network that establishes a selectively variable reference voltage which controls the amount of current through the lamp. The fullwave rectifier bridge supplies a small amount of power to the control circuitry. The current limiting circuit allows continuous dimming control from full light output to zero output of both arc discharge lamps and incandescent lamps by limiting the current through the lamp without significantly reducing the electrical conduction angle of the alternating current power supply below 360 degrees and will also control dimming of systems powered by direct current.